The present invention relates to processes for making papermaking belts. More particularly, the present invention is concerned with a method of making a belt comprising a reinforcing structure and a resinous framework joined thereto.
Generally, through-air-drying papermaking processes include several steps. An aqueous dispersion of papermaking fibers is formed into an embryonic web on a foraminous member, such as Fourdrinier wire. This embryonic web is associated with a deflection member having a macroscopically-planar, and preferably non-randomly patterned network surface which defines within the deflection member a plurality of fluid-permeable deflection conduits. In a continuous papermaking process, this deflection member is in the form of an endless papermaking belt. If the patterned network surface of the deflection member is substantially continuous, the plurality of deflection conduits comprises discrete conduits isolated from one another. If the patterned network surface of the deflection member comprises a plurality of isolated from one another protuberances, the plurality of conduits form a substantially continuous area. The papermaking fibers are deflected into the deflection conduits, and water is removed through the deflection conduits, thereby an intermediate web is formed. The intermediate web is then dried and, if desired, is foreshortened by creping.
The papermaking belt comprising the deflection member is described in several commonly-assigned U.S. Pat. No. 4,514,345, issued Apr. 30, 1985 to Johnson et al.; U.S. Pat. No. 4,528,239, issued Jul. 9, 1985 to Trokhan; U.S. Pat. No. 5,098,522, issued Mar. 24, 1992; U.S. Pat. No. 5,260,171, issued Nov. 9, 1993 to Smurkoski et al.; U.S. Pat. No. 5,275,700, issued Jan. 4, 1994 to Trokhan; U.S. Pat. No. 5,328,565, issued Jul. 12, 1994 to Rasch et al.; U.S. Pat. No. 5,334,289, issued Aug. 2, 1994 to Trokhan et al.; U.S. Pat. No. 5,431,786, issued Jul. 11, 1995 to Rasch et al.; U.S. Pat. No. 5,496,624, issued Mar. 5, 1996 to Stelljes, Jr. et al.; U.S. Pat. No. 5,500,277, issued Mar. 19, 1996 to Trokhan et al.; U.S. Pat. No. 5,514,523, issued May 7, 1996 to Trokhan et al.; U.S. Pat. No. 5,554,467, issued Sep. 10, 1996, to Trokhan et al.; U.S. Pat. No. 5,566,724, issued Oct. 22, 1996 to Trokhan et al.; U.S. Pat. No. 5,624,790, issued Apr. 29, 1997 to Trokhan et al.; U.S. Pat. No. 5,628,876 issued May 13, 1997 to Ayers et al.; U.S. Pat. No. 5,679,222 issued Oct. 21, 1997 to Rasch et al.; and U.S. Pat. No. 5,714,041 issued Feb. 3, 1998 to Ayers et al., the disclosures of which patents are incorporated herein by reference.
Deflection of the papermaking fibers into the deflection conduits of the deflection member can be induced by, for example, application of differential fluid pressure to the embryonic web. One preferred method of applying the differential pressure comprises exposing the embryonic web to a vacuum through the deflection conduits of the deflection member. A sudden application of the differential fluid pressure to the fibers tends to separate some of the fibers deflected into the deflection conduits from the web and from one another. In addition, as a result of a sudden application of the pressure differential to the web, a certain number of partially dewatered fibers separated from the embryonic web could completely pass through the conduits of the deflection member. These phenomena cause formation of so-called xe2x80x9cpinholesxe2x80x9d in the finished paper web, and also may lead to clogging of the vacuum dewatering machinery-with the fibers separated from the web.
This undesirable creation of the pinholes may be mitigated by providing a leakage between the deflection member and the papermaking equipment creating the fluid pressure differential. One way of creating such a leakage comprises providing surface texture irregularities in the backside network of the deflection member. Commonly-assigned U.S. Pat. No. 5,098,522 issued Mar. 24, 1992 to Smurkoski et al.; and U.S. Pat. No. 5,364,504 issued Nov. 15, 1994 to Smurkoski et al. disclosed a process of making a backside textured belt, comprising casting a photosensitive resinous material over and through a reinforcing structure while the reinforcing structure travels over a textured working surface. Commonly-assigned U.S. Pat. No. 5,275,700, issued Jan. 4, 1994 to Trokhan et al. disclosed a process of making a backside textured belt, comprising applying a coating of a photosensitive resin to the reinforcing structure and pressing the reinforcing structure into a deformable surface so that the deformable surface forms protrusions which exclude resin from certain areas, thereby creating a textured backside of the belt. Commonly-assigned U.S. Pat. No. 5,334,289 issued Aug. 2, 1994 to Trokhan et al. discloses a process for making a backside textured belt, comprising applying a coating of photosensitive resin to a reinforcing structure which has opaque portions, and then exposing the resin to curing radiation through the reinforcing structure. The foregoing patents are incorporated herein by reference.
Another way of mitigating the undesirable creation of the pinholes, developed by the present assignee, comprises making the belt having deflection conduits such that a substantial portion of each conduit is not less than about 45 mils in each of the conduit""s dimensions (measured in the general plane of the belt, i.e., an X-Y plane), as disclosed in commonly-assigned U.S. Pat. No. 5,679,222 issued Oct. 21, 1997 to Rasch et al., which is incorporated herein by reference.
Still, the search for improved products has continued.
The present invention provides a novel process for making an improved papermaking belt comprising a reinforcing structure and a resinous framework joined thereto. It is a benefit of the present invention to provide a novel process for making a backside-textured papermaking belt. Another benefit of the present invention is that it provides a novel process for making a papermaking belt in which the depth of penetration of the resin into the reinforcing structure is controllable such that the resin penetrates a predetermined portion of the thickness of the reinforcing structure, so as to provide acceptable bonding of the resin to the reinforcing structure, while maintaining the flexibility of the reinforcing structure, as well as the permeability to air and water. Still another benefit of the present invention is that it provides a process for reducing the amount of the resinous material required for making the belt.
A papermaking belt that can be made by a process and an apparatus of the present invention comprises a reinforcing structure and a patterned resinous framework joined thereto. The reinforcing structure has a first side and an opposite second side. The resinous framework has a top side and a bottom side, the top and bottom sides corresponding to the first and second sides of the reinforcing structure, respectively. The resinous framework and the reinforcing structure are joined together such that a substantial portion of the bottom side of the resinous framework is elevated over the second side of the reinforcing structure. That is, the belt has a distance formed between the second side of the reinforcing structure and the substantial portion of the bottom side of the resinous framework. During a papermaking process, this distance provides leakage between the belt and a dewatering papermaking equipment, thereby eliminating a sudden application of fluid pressure differential to a paper web disposed on the belt and mitigating a phenomenon known as xe2x80x9cpinholling.xe2x80x9d The distance between the second side of the reinforcing structure and the bottom side of the resinous framework may differentiate throughout the plane of the belt.
An apparatus for making the papermaking belt comprises a working surface having a pattern of recesses therein, a means for disposing the reinforcing structure in the pattern of recesses of the working surface, a means for applying a fluid resinous material to the reinforcing structure disposed in the recesses of the working surface, and a means for solidifying the fluid resinous material to provide a patterned resinous framework joined to the reinforcing structure. The pattern of recesses is structured and designed to at least partially receive the reinforcing structure therein. In a preferred continuous process, the apparatus further comprises a means for continuously moving the reinforcing structure in a machine direction.
A process for making the belt comprises the following steps. A fluid resinous material is provided. The fluid resinous material is preferably selected from the group consisting of epoxies, silicones, urethanes, polystyrenes, polyolefins, polysulfides, nylons, butadienes, photopolymers, and any combination thereof. In one preferred embodiment the fluid resinous material comprises a photosensitive resin. In another preferred embodiment, the fluid resinous material comprises a thermo-sensitive resin. Preferably, the fluid resinous material is provided in a liquid state.
The next step comprises providing a reinforcing structure having a first side, a second side opposite to the first side, and a thickness defined therebetween. Preferably, a fluid-permeable reinforcing structure is used, such as, for example, a woven reinforcing structure. However, the use of the reinforcing structure that is not fluid-permeable is also contemplated in the present invention. Preferably, the reinforcing structure has voids intermediate its first and second sides, which voids are penetrable by the fluid resinous material.
The next step comprises providing a working surface. The working surface comprises an external surface and a pattern of recesses therein. The recesses have a depth and are structured and designed to receive the reinforcing structure therein.
The next step comprises at least partially disposing the reinforcing structure in the recesses of the working surface. Various means, such as support rolls and press rolls, may be used to facilitate disposing the reinforcing structure in the recesses. Because of the flexible nature of the reinforcing structure, the reinforcing structure can be only partially disposed in the recesses. In some embodiments, the reinforcing structure is disposed in the pattern of recesses to extend beyond the external surface of the working surface. In these instances, it is preferred that at least a portion of the voids penetrable by the fluid resinous material extends beyond the external surface of the working surface.
The next step comprises applying the fluid resinous material to the reinforcing structure. Preferably, the resinous material occupies at least some of the voids in the reinforcing structure such as to xe2x80x9clock on,xe2x80x9d or xe2x80x9cencase,xe2x80x9d portions of the reinforcing structure as to effectively form a bond therebetween. Alternatively, the resinous material may attach to the reinforcing structure, without locking on around the portions thereof. In the latter case, the first side of the reinforcing structure preferably possesses sufficient roughness or/and be adhesive, which can be achieved by special treatment of the reinforcing structure.
The fluid resinous material may be applied to the reinforcing structure in the form of a substantially uniform layer, orxe2x80x94alternativelyxe2x80x94according to a predetermined pattern. The former embodiment is particularly relevant for the photosensitive curable resinous materials, while the latter embodimentxe2x80x94for the processes using a patterned molding surface to deposit the resinous material onto the reinforcing structure. In the latter instance, the process further comprises the steps of providing a patterned molding surface juxtaposed with the reinforcing structure, and depositing the fluid resinous material into the molding pockets of the molding surface. The molding surface has a pattern of molding pockets therein, which pockets are structured and designed to receive the fluid resinous material and then to deposit it onto the reinforcing structure. The plurality of molding pockets may comprise a substantially continuous pattern, a pattern of discrete pockets, or a semi-continuous pattern comprising a combination of the substantially continuous pattern and the pattern of discrete pockets. The molding surface may comprise a surface of a rotatable molding roll, or a surface of an endless molding band.
The next step comprises solidifying the fluid resinous material to form a patterned resinous framework joined to the reinforcing structure. The nature of the resinous material dictates a method of its solidifying. Preferably, the solidifying comprises a curing (i.e., a process involving cross-linking) of the resinous material. The photosensitive resinous materials can be cured by a curing, typically UV, radiation. Some thermo-sensitive resinous materials can cure naturally, during a certain period of time. After the resinous material has solidified, it forms a resinous framework securely joined to the reinforcing structure, thus forming the papermaking belt.